Multiple Personality Disorder (MPD), also known as Dissociative Identity Disorder (DID), is a psychiatric disorder characterized by having at least one "alter" personality that controls behavior. These personalities are said to occur spontaneously and involuntarily. Symptoms include the inability to recall key personal information, mood swings, depression, suicidal tendencies, sleep disorders, anxiety panic attacks, alcohol and drug abuse, compulsions and rituals, hallucinations, and eating disorders.
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The causes of MPD are generally triggered by childhood trauma, often physical or sexual abuse. Often the memory of the abuse has been suppressed and must be recovered through psychotherapy. Recently, it has been shown that psychotherapists, without realizing it, can lead a person to invent "suppressed" memories. This has led to harm to innocent people. This is not a genetically inherited disorder. MPD is also triggered mainly by other living things around the person.
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There are many public views on MPD that are incorrect. People may think that everyone with MPD is a dangerous threat to society. This is false. There are many cases in which people have MPD and the second personality is just as safe as the first original one. People also think that MPD sufferers are just a bunch of "whack jobs". This is also false. These people have gone through a traumatic experience when they were children that caused them to do this and only need help to get over their problem.
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Psychotherapists generally considered to be the main component of treatment for MPD. Therapists try to help clients improve their relationships with others and to experience feelings they have not felt comfortable expressing in the past. Mental Health Professionals also often help guide the MPD sufferer to find a way for his personalities to coexist and work together. Another way to help these people is Hypnosis. Hypnosis is used to help increase the information that the person with MPD has about their symptoms or their others personality, which increases the control they have over their other personality when the switch from one to the other. Medications are often used to address the symptoms of this issue like depression or anxiety.
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Bipolar is often confused with MPD. People see the drastic change in mood and behavior and think it may be related to two different personalities. Schizophrenia is often associated with MPD because of the delusions and hallucinations in both the disorders.

Schizophrenia
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Book Source: "Personality Disorders" in Diseases and Disorders. Published by Marshall Cavendish, New York.